0.001 Gold is equal to 5 cents. I think that it is more than fair minimal fee for storing gold. 5$ minimum transaction also seems to be fair. Even exchanges like binance has minimum trade size set to 0.001 BTC (~7$).
If both of those information are in TOS than I don't see more fair solution. We are buying physical gold. I would not expect that someone will process transactions for the amount of gold that can be hidden under the nail.